314 THE ICE AGE IN NORTH AMERICA. 



reached the latitude of New York city, the valleys of the St. 

 Lawrence and of the Mohawk must have been closed up by 

 ice so as to reverse their lines of drainage. The waters of 

 Lakes Superior and Michigan must then have flowed into the 

 Mississippi River along the lines of the Fox, Wisconsin, and 

 Illinois Rivers, while those of Lakes Huron and Erie poured 

 into the Ohio River, at first down the Wabash, then, a little 

 later, when the extension of the central lobe of ice cut off the 

 western outlet of Lake Erie, over the lowest places in the 

 water-shed into the valleys of the Scioto, the Muskingum, 

 and Beaver Rivers ; at the same time, every northern tribu- 

 tary of the Alleghany was a glacial flood. 



But the scenes to have been witnessed during the ad- 

 vance of the ice-sheet are as nothing compared with those 

 which must have occurred during its retreat. Even now, 

 every spring has its freshets, when the combined action of ice 

 and water produces floods unparalleled at other seasons of the 

 year. If this is the case upon the melting of the small amount 

 of snow which annually accumulates in our present winters, 

 the floods at the breaking up of the Glacial period itself 

 must have been inconceivably great. With every recurring 

 spring we now look in the telegraphic summary for thrilling 

 accounts of ice-gorges formed in the St. Lawrence, the Dela- 

 ware, the Susquehanna, and the Missouri River. By reason 

 of these gorges, and their accompanying destructive floods, 

 Port Jervis, on the Delaware, and Mandan, on the Missouri, 

 have become familiar names. Reasoning from the nature of 

 the case, what, then, must have been the scenes during the 

 last stages of the great Ice age, when, through the months 

 of July, August, and September, warm southerly winds and 

 a glowing sun were combining to dissolve, with utmost ra- 

 pidity, the vast masses of ice which still lingered in the 

 country ! The channels were then compelled to carry off 

 not only the annual precipitation, and the torrents of an oc- 

 casional cloud-burst, but the stored-up precipitation which 

 had been accumulating as glacial ice for thousands of years. 



Nor is this altogether theoretical. Though we have no 



